1. Field of the Invention
The present invention relates to a broad-band radio wave absorber and, more particularly, to a radio wave absorber made of a ferrite as a magnetic substance, in which its frequency range is made broader. The broad-band radio wave absorber is extensively employed as a material for a wall of a radio wave anechoic chamber adapted so as to measure a radio wave radiated from electronic devices and for a wall for a prevention of the reflection of a radio wave for television from buildings.
2. Description of the Related Art
Heretofore, as a method for making the frequency range of a radio wave absorber composed of a single layer of a ferrite broader, there has been proposed a method in which a ferrite as in a form of tiles is so disposed as to be away from a reflecting plate, for example, a polyurethane foam plate or the like, containing a layer of air.
For instance, when 7 mm-thick tiles of a sintered NiZn ferrite are placed in a spaced relationship away in 8 mm to 15 mm from a reflecting plate, a broad-band radio wave absorber can be prepared which has a reflectivity of -20 dB or smaller for the frequency band of 30 MHz to 1,000 MHz. On the other hand, broad-band radio wave absorbers of a fin type and of a lattice type have been proposed by the present inventors. Of the broad-band radio wave absorbers proposed, the broad-band radio wave absorber of the lattice type, prepared by making a sintered NiZn ferrite in a form of a lattice of 7 mm thick and 20 mm high, may have a reflectivity of -20 dB or lower for a range of 30 MHz to 700 MHz.
FIG. 18 is a sectional view showing a typical structure of a radio wave absorber of a ferrite type in which a plate F of a sintered ferrite as in a tile form is arranged on a conductive metal plate M for reflecting the radio wave. When the reflection coefficient of on a surface of the ferrite absorber is indicated by "s", the power absorption coefficient of the wave absorption body is given by the following formula (1): EQU 1-.vertline.s.vertline..sup.2
Thus, it can be said that the smaller the reflection coefficient .vertline.s.vertline. the better the absorber.
It is noted herein that a standard is usually set up by the following formula (2): EQU .vertline.s.vertline..ltoreq.0.1
In other words, the reflectivity is set to be equal to or smaller than -20 dB (20 log s dB), and the absorption coefficient is set to be equal to or larger than 0.99.
The characteristics of the radio wave absorber as shown in FIG. 18 may be indicated as shown in FIG. 19 in which the x-axis is set by frequency f and the y-axis is set by the magnitude of reflection coefficient .vertline.s.vertline.. In this case, when the lower of two frequencies for which .vertline.s.vertline.=0.1 is fL and the higher is fH, then the band width B for which .vertline.s.vertline..ltoreq.0.1 is satisfied becomes (3): EQU B=fH-fL
A further description will be made of the band width of the frequency.
When the lower limit frequency fL is set to 30 MHz, the ferrite to be employed is of a sintered NiZn type or of a sintered MnZn type. In this case, the resulting absorber may generally give the upper limit frequency fH of 300 MHz to 400 MHz.
On the other hand, the lower limit frequency fL is set to 90 MHz. In this case, the resulting absorber may generally give the upper limit frequency fH of 350 MHz to 520 MHz.
If the ferrite of the sintered NiZn type or of the sintered MnZn type is to be employed as the wall material for the radio wave darkroom for measuring the radio wave radiated from electronic devices, the upper limit frequency fH is usually required to satisfy fH=1,000 MHz when the lower limit frequency fL is set to 30 MHz. Hence, the ferrite as exemplified hereinabove does not satisfy the characteristic for the upper limit frequency.
On the other hand, if the ferrite of the sintered type is to be employed as the material for the wall for the purpose to prevent a reflection of the radio wave for television from buildings, the upper limit frequency fH is usually required to satisfy fH=800 MHz, when the lower limit frequency fL is set to 90 MHz. In this case, the ferrite as exemplified hereinabove is insufficient in terms of the upper limit frequency.
Furthermore, proposals have been made of improvements in the broad-band radio wave absorber of such a type as shown in FIG. 18.
One proposal for making a band width of the radio wave absorber broader is such that an air layer, a dielectric material or a loss dielectric material, as indicated by reference symbol "D", is interposed between ferrite F and a metal plate M as shown in FIG. 20. This broad-band radio wave absorber, however, can provide the lower limit frequency fL of 30 MHz and the upper limit frequency fH of 1,000 MHz at the most.
Another proposal for making the band wider is made by the present inventors, as disclosed in Japanese Patent Application No. 162,403/1990, that sintered ferrites are disposed at a predetermined interval S on a conductive metal plate, each ferrite having a height, as indicated by reference symbol h, and a thickness, as indicated by reference symbol t, the height h being larger than the thickness t, as shown in FIGS. 21A, 21B and 22. For brevity of description, the arrangement for the sintered ferrites as shown in FIGS. 21A and 21B will be hereinafter referred to as a fin-type radio wave absorber and the arrangement for them as shown in FIG. 22 will be hereinafter referred to as a lattice-type radio wave absorber. The lattice-type radio wave absorber is adapted for a bidirectional polarization of an incident wave. On the other hand, the fin-type radio wave absorber is structured in such a manner that the ferrites extending laterally or transversely are removed from the lattice-type radio wave absorber and it is adapted for a unidirectional polarization of an incident wave; however, its basic operation is the same as that of the lattice-type radio wave absorber. For instance, the fin-type radio wave absorber as shown in FIGS. 21A and 21B gives the upper limit frequency fH=2,400 MHz, and the lattice-type radio wave absorber as shown in FIG. 22 gives the upper limit frequency fH=700 MHz to 1,500 MHz.
It is anticipated that, as a higher frequency will be needed for operating electronic devices and, as a result, the frequency of a radio wave radiated will become higher, a higher upper limit frequency fH will be required as a matter of course.
Further, recently, an interest in EMI (ElectroMagnetic Interference) is growing so that a radio wave absorber having a broader band is desired.